A cellulose ester resin (CA) film has been used as a polarizer protective film constituting a polarizing plate of a liquid-crystal display device such as televisions and notebook computers from the viewpoints that it has good transparency, optical isotropy, and toughness, and good adhesiveness to polyvinyl alcohol (hereinafter abbreviated as “PVA”) which is a material of a polarizer of a liquid-crystal display device.
The liquid-crystal display device has been required to have a viewing angle improving function. In particular, it has hitherto been promoted that a liquid-crystal display device in a vertically aligned (VA) mode device performs compensation of a viewing angle by overlapping a phase difference film with a polarizer protective film in order to prevent a reduction in contrast due to light leakage in a case of being viewed in an oblique direction. In addition, recently, a polarizer protective film having a phase difference function, in which the functions of two films, a polarizer protective film and a phase difference film, are integrated into one film in order to reduce the weight and the thickness of a liquid-crystal display device, is the mainstream.
The polarizer protective film having a phase difference function expresses a phase difference in the thickness direction due to its optical anisotropy to perform viewing angle compensation of a liquid-crystal display device. Generally, it is possible to determine the phase difference degree through a retardation value. Further, the phase difference degree can be evaluated through a retardation value (hereinafter abbreviated as an “Rth value”) in the thickness direction of a film.
Here, the Rth value in the thickness direction is a value defined by the following equation (1).Rth Value={(nx+ny)/2−nz}×d(nm)  (1)
(in the equation, nx is a refractive index in the slow phase axis direction within the film plane, ny is a refractive index in the fast axis direction within the film plane, nz is a refractive index in the thickness direction of the film, and d is the thickness (nm) of the film).
In a case of imparting a phase difference function to a polarizer protective film, a technique in which a so-called retardation enhancer is added to the polarizer protective film so as to make an adjustment to a desired phase difference is available, and it is possible to adjust the Rth value of the polarizer protective film, depending on the amount of the retardation enhancer to be added to the polarizer protective film. Accordingly, in a case of comparison of the same addition amounts, a retardation enhancer capable of increasing the Rth value to a higher value broadens a range to which the Rth values of the polarizer protective film can be adjusted, and thus, can cope with a decrease in the thickness of the polarizer protective film. Thus, there is a demand for a material which can increase the Rth value up to a maximum.
For the adjustment to a desired phase difference, a method of selecting the kind of a cellulose ester resin which forms a main agent, in addition to the use of the retardation enhancer, is also available. Specifically, a method for making an adjustment to a desired phase difference by using a modified cellulose ester resin such as cellulose acetate propionate (CAP), or a cellulose ester resin having a low degree of substitution of an acyl group, such as diacetyl cellulose (hereinafter referred to as CA having a low acetylation degree) among the cellulose ester resins is known. In particular, the CA having a low acetylation degree has an excellent Rth expression property and is more cost-effective than CAP. From these advantages, CA having a low acetylation degree has been used as a main agent of the polarizer protective film having a phase difference function.
However, the CA having a low acetylation degree has a problem in high polarity, and correspondingly, low compatibility with the retardation enhancer, and therefore, there has been a limit on the retardation enhancer to be used. Accordingly, there has been a strong demand for a retardation enhancer having good compatibility with CA having a low acetylation degree.
Furthermore, it is necessary for the polarizer protective film to have bleed resistance while not deteriorating the clarity of an image due to occurrence of cloudiness caused by the bleeding of additives from the surface of the polarizer protective film by the heat of a backlight in a liquid-crystal display device or by the use of a liquid-crystal display device under a high temperature and a high humidity.
As the retardation enhancer which has excellent bleed resistance and can impart a high Rth value, for example, an ester compound having an end sealed with p-toluic acid, which is obtained by esterification of 1,2-propylene glycol, dimethyl terephthalate, and p-toluic acid, is known (see, for example, PTL 1). However, even though the ester compound disclosed in PTL 1 was used, a film having a sufficiently high Rth value could not be obtained.
Moreover, as a retardation enhancer which has excellent bleed resistance and can impart a high Rth value as in PTL 1, an ester compound obtained by using dimethyl 2,6-naphthalenedicarboxylate, propylene glycol, and ethylene glycol is known (see, for example, PTL 2). However, the ester compound disclosed in PTL 2 has a problem in that it has insufficient compatibility with CA having a low acetylation degree, and as a result, cloudiness occurs in the obtained film.
In addition, as a retardation enhancer which has excellent bleed resistance and can impart a high Rth value as in PTLs 1 and 2, a tetramethyl biphenol-type epoxy ester compound obtained by reacting a tetramethyl biphenol-type epoxy resin with an aromatic monocarboxylic acid is known (see, for example, PTL 3). However, the epoxy ester compound disclosed in PTL 3 has a problem in that it has insufficient compatibility with CA having a low acetylation degree, and as a result, cloudiness occurs in the obtained film as the ester compound as disclosed in PTL 2.